Multimedia Broadcast Multicast Service (MBMS) is described in various standards such as the Third Generation Partnership Project (3GPP), Universal Mobile Telephone System (UMTS) standards. More recently, MBMS functionality has been specified in relation to orthogonal frequency division multiplexing (OFDM) physical layers, such as those exemplified by the IEEE 802.16e-2005 amendment or 3GPP Long Term Evolution (LTE) project (to which the term Evolved MBMS or EMBMS is sometimes applied). MBMS over OFDM and other selected physical layer structures (of which EMBMS is an example) enables a multicast approach referred to as multicast broadcast single frequency network (MBSFN). OFDM is particularly favorable for MBSFN operation when the OFDM symbols contain a cyclic prefix (CP) that can constructively recover multiple signal paths from one or more transmission sources. MBSFN techniques operate by simulcasting (i.e. transmitting on identical time-frequency network resources) from various cells (base transceiver stations or “enhanced node Bs”, eNB's), over a given geographic area. Such areas may be defined by the radio coverage area of a group of base transceiver stations or even smaller areas as defined by antenna coverage sectors of the base transceiver station. Sometimes the term single frequency network area (SFA) is defined as the set of cells participating in the simulcasting procedure.
In addition to receiving MBMS service (where the generic term MBMS is understood to include 3GPP LTE-specific EMBMS), a mobile station, also referred to as “User Equipment” or “UE,” may also engage in communication using dedicated channels or other via unicast signaling where the UE communicates with a specific base transceiver station.
Receiving unicast information by a UE is distinguishable from MBMS delivery reception, because the MBSFN is generally broadcast from one or more base transceiver stations (i.e. the SFA) such that a UE receives one composite desired signal from the base transceiver stations. Therefore, the individual base transceiver station transmissions are indistinguishable to the UE in the case of MBMS reception. Note that in the current context, while MBSFN transmission from multiple base stations is expected to be a typical operating mode, transmission from a single base station is specifically recognised as a practical case.
However a UE must have the ability to make measurements of individual base stations both for mobility and link adaptation purposes. In the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) core specifications base transceiver stations may be defined with up to 4 antenna ports. A set of associated reference symbols (RS, numbered RS 0, 1, 2, 3) may be defined and applied to Multiple Input, Multiple Output (MIMO) transmission of unicast downlink traffic. However, the UE is generally required to access only RS 0 and 1 for mobility measurement purposes, and may further need to access only RS 0 and 1 for reception of any unicast transmission from a specific cell delivered in coordinated fashion with the MBSFN transmission.
As specified by 3GPP, a downlink subframe formatted for MBMS transmission using the multicast broadcast single frequency network approach (MBSFN) over the Multicast Channel (MCH) is applied by time-division multiplexing (TDM) unicast and MBSFN subframes (note that here, the terms MCH and MBSFN subframe are used to indicate a subframe simulcast by one or most base stations). In at least one configuration, in any MBSFN subframe region associated with control data, only RS 0 and RS 1 will be present and transmitted in unicast mode by each base station, and any reference symbols transmitted over the MBSFN or MCH portion of the subframe may not be used as a phase reference for demodulating MIMO unicast transmissions or for demodulating any subsequent unicast subframe (i.e. a sub-frame dedicated to use by the so-called downlink shared channel (DL-SCH) in the case of 3GPP LTE). Nor may reference symbols transmitted over the MBSFN be used for other purposes related to the cell-specific unicast transmission, such as link adaption or MIMO transmission rank adaption.
Accordingly, given the adopted TDM approach to MBSFN structure in 3GPP LTE (and similar systems), and unicast allocated subframes, it would not be possible to maintain appropriate link adaptation metrics for all possible (up to 4) MIMO streams for a unicast subframe following a broadcast subframe due to the absence of RS 2 and 3 during a MBSFN-allocated (i.e. MCH-allocated) subframe.
Therefore, a means is required to enable observation of, and link adaptation in consideration of, the 3rd and 4th antennas (antennas numbered 2 and 3) while receiving MCH-allocated subframes or at least to minimize the impact of not observing antennas 2 and 3 while receiving MBSFN prior to re-commencement of non-MBSFN transmissions.